Adsorption and photocatalytic oxidation of formaldehyde on a clay-TiO2 composite

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Abstract

We investigated the adsorption capacity and photocatalytic removal efficiency of formaldehyde using a hectorite-TiO2 composite in a bench flow reactor. The same experimental conditions were applied to pure TiO2 (Degussa P25) as a reference. The catalysts were irradiated with either a UVA lamp (365 nm) or with one of two UVC lamps of 254 nm and 254 + 185 nm, respectively. Formaldehyde was introduced upstream at concentrations of 100–500 ppb, with relative humidity (RH) in the range 0–66% and residence times between 50 and 500 ms. Under dry air and without illumination, saturation of catalyst surfaces was achieved after ∼200 min for P25 and ∼1000 min for hectorite-TiO2. The formaldehyde uptake capacity by hectorite-TiO2 was 4.1 times higher than that of P25, almost twice the BET surface area ratio. In the presence of humidity, the difference in uptake efficiency between both materials disappeared, and saturation was achieved faster (after ∼200 min at 10% RH and ∼60 min at 65% RH). Under irradiation with each of the three UV sources, removal efficiencies were proportional to the Ti content and increased with contact time. The removal efficiency decreased at high RH. A more complete elimination of formaldehyde was observed with the 254 + 185 nm UV source.

Highlights

► Formaldehyde adsorption and photocatalytic elimination on hectorite-TiO2 nanocomposites. ► Dark adsorption in dry air >4 times higher than P25 (reference). ► Dark adsorption in humid air dominated by adsorbed water layer. ► Photocatalytic removal efficiency proportional to the Ti content, increased with contact time. ► More complete elimination with 254 + 185 nm irradiation.

Introduction

Formaldehyde (HCHO) is an ubiquitous indoor pollutant released from wood-based building products and furnishings, among other natural and anthropogenic sources [1]. Formaldehyde forms as a result of the oxidation of volatile organic compounds (VOCs) by either ozone or hydroxyl (OHradical dot) radicals under atmospheric conditions [2], [3]. Indoor exposure to HCHO is associated with increased risks of asthma and allergy [4]. Reportedly, observed changes in nasal lavage fluids during formaldehyde inhalation have been attributed to non-specific proinflammatory properties [5]. Asthma and allergies are reported to affect ca. 6% and 20% of the 89 million US workers in nonagricultural and nonindustrial indoor settings, respectively. Such health consequences stemming from formaldehyde inhalation in the workplace has been reported to cause productivity losses ranging from 20 to 70$B yr−1 [6]. Furthermore, formaldehyde is listed by USEPA as a probable carcinogen (group B1, USEPA), and the World Health Organization has classified formaldehyde as a human carcinogen [7]. Surveys conducted in both US commercial buildings and homes showed mean indoor HCHO concentrations values ca. 11 and 17 ppbv, respectively. Such concentrations are higher than the 8-h reference exposure levels proposed by the California Environmental Protection Agency, i.e., 7 ppbv, and are close to the 8-h recommended level for occupational exposure in the US (16 ppbv) [8]. Current indoor-air pollutant exposure scenarios are likely to worsen in a near future provided that adaptation to climate change and urban heat island effects may lead to increases in the use of air conditioning, tighter building envelopes, as well as to lower air-exchange rates [9]. In addition, expanding urbanization and changes in land use patterns may contribute to increased surface-level concentrations of ozone, an indoor formaldehyde precursor [10].

Advanced indoor air cleaning technologies can play an important role in mitigating indoor exposures. In a related work we have tested prototype TiO2 photocatalytic oxidation (PCO) air cleaners. The results have showed promise in the simultaneous abatement of VOCs present in multi-component mixtures at typical indoor levels [11], [12], [13]. We observed single-pass conversion efficiencies better than 20% for most VOCs, reaching in some cases as much as 80% removal. Although volatile aldehydes can be eliminated by PCO at rates comparable to those for other VOCs, incomplete mineralization of a few target compounds present in the mixtures (alcohols, terpenes) results in the formation of additional HCHO, acetaldehyde, and other partially oxidized byproducts. For the experimental conditions tested, HCHO outlet/inlet concentration ratios were between 1.9 and 7.2. Given the data variability observed, it becomes clear the need for improving experimental conditions towards PCO applications.

Clay-TiO2 nanocomposites have been postulated as suitable alternative photocatalysts in environmental applications. In particular, for air treatment considerations, these materials offer a large porous structure for VOC adsorption and high adsorption capacity. Recently, we have synthesized hectorite-TiO2 composite (hecto-TiO2) [14], a titania-rich material (60% TiO2) with significantly higher BET surface than Degussa P25 TiO2 (BEThectoTiO2/BETP25=2.3). We tested the material towards toluene as probe compound. When challenged with toluene vapor, hecto-TiO2 showed a performance comparable to P25 under air either under dry conditions or low relative humidity, ca. ≤10% RH. However, hecto-TiO2 performance was found to become partially inhibited at higher humidity, ca. 33% and 66% RH [15]. These findings were explained as the consequence of water adsorption and condensation at nano-sized pores sites, which limits the access of hydrophobic compound molecules to TiO2 active sites. In this study, we challenge hecto-TiO2 under similar testing conditions with HCHO, a hydrophilic compound. The purpose is to explore the photocatalytic activity of surface clay-TiO2 composites towards HCHO, and better understand the effect of water co-adsorption in photocatalytic efficiency.

Section snippets

Preparation of clay-supported TiO2

Hectorite (Na0.4Mg2.7Li0.3Si4O10(OH)2; SHCa-1) from San Bernardino County, CA, USA, was purchased from the Source Clays Repository of the Clay Minerals Society (West Lafeyette, IN), and used as received. A description of the synthesis and characterization of the TiO2–clay nanocomposites has been reported previously [14]. Briefly, a 1% (w/w) clay–water suspension was stirred for 2 h. A TiO2 sol–gel solution was prepared by mixing titanium tetraisopropoxide Ti(OC3H7)4 (97%, Sigma–Aldrich,

Formaldehyde adsorption in the absence of illumination

We studied the adsorption of HCHO to each of the photocatalysts in the dark, at different humidity conditions. These tests allowed us to determine the minimum time needed to complete HCHO uptake and saturation of the photocatalyst surface, in order to perform subsequent experiments by irradiating with UV light. Equilibrium saturation was reached when similar HCHO concentrations were measured simultaneously at the reactor inlet and outlet. As shown in Fig. 2 for experiments conducted with dry

Conclusions and implications

We evaluated the performance of hectorite-TiO2 nanocomposites with respect to the reference material P25 under controlled conditions of relative humidity, UV irradiation and residence time, using formaldehyde as a target compound. Overall, the clay-TiO2 composite showed comparable efficiency in the removal of the model pollutant when normalized by the mass content of TiO2. The observed influence of key experimental parameters on clay-TiO2 is consistent with that observed on P25 and with

Acknowledgments

This work was supported by Laboratory Directed Research and Development (LDRD Project #08-103, LB07014) funding from Lawrence Berkeley National Laboratory, provided by the Director, Office of Science, of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231. J.C.S. thanks B.Sc. Cesar Saavedra Alamillas, M. in Sc. María del Rocío Galindo Ortega (Universidad Autónoma Metropolitana Unidad Cuajimalpa), M. in Sc. Pilar Fernández Lomelin (Instituto de Geografía, UNAM) for technical

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